The ability to process polymers to desired shapes and sizes is one of the properties that makes polymers highly desirable for countless industrial and commercial applications. As with all materials, cost saving methods for processing and utilizing polymers are of key importance. One method for reducing the cost of a polymer article is through the use of blowing agents which produce a foam, thereby reducing the amount of polymer per unit volume. Unfortunately, conventional foamed polymers tend to exhibit a decrease in the mechanical properties.
Microcellular foams have been made in the past by foaming amorphous polymeric materials such as polystyrene. This process involves cell nucleation at or near the glass transition temperature of the polymer material and is more thoroughly described in U.S. Pat. No. 4,473,665 of Martini-Vvendensky, et al. This patent, however, does not teach foaming of semi-crystalline polymers such as polyethylene and polypropylene.
Attempts to produce microcellular foamed articles from semi-crystalline polymers using the method employed for amorphous polymers have proven to be failures. The three basic problems arising when microcellularly foaming semi-crystalline polymers result from the polymers' microstructure. They are as follows:
1) low gas solubility in the crystalline domains, PA1 2) requirement to foam near the melting temperature rather than near the glass transition temperature, and PA1 3) the physical size and structure of the crystals. PA1 a) saturating the semi-crystalline polymeric material at an elevated pressure and at a temperature at or above the melting temperature of the material with a uniform concentration of gas; PA1 b) shaping the polymeric material at an elevated pressure to substantially prevent cell nucleation within the material; PA1 c) reducing the pressure to supersaturate the polymeric material with gas and thereby produce a microcellular structure in said material, and PA1 d) lowering the temperature below the melting point of said polymeric material.
Taking the above characteristics of semi-crystalline polymers into account, it is apparent that conventional microcellular foaming techniques will fail to provide a desirable product when used on these polymer types.